Abscisic acid (ABA) is an important phytohormone that participates in plant abiotic stress signaling. In some plants, endogenous ABA is produced in response to environmental stresses, such as heat stress, water stress, and salt stress. Under these conditions, an increase in ABA triggers a pathway that ultimately results in an increased expression of stress-tolerance related genes. These stress-tolerance related genes are typically inhibited by enzymes called clade-A protein phosphatases type 2Cs (PP2Cs, comprising ABI1, ABI2, HAB1, HAB2, AHG1, PP2CA, HAI1, HAI2, and HAI3), but the inhibition is released and the stress-tolerance related genes are expressed in the presence of ABA and a family of PYR/PYL/RCAR proteins (hereafter referred to as PYLs).
The PYLs are ABA receptors in the cytoplasm and nucleus (Fujii et al., 2009; Ma et al., 2009b; Park et al., 2009). The PYL family has 14 members in Arabidopsis, including PYR1 and PYL1-13, each containing a START domain. Several of the PYLs have been shown to bind to and inhibit PP2Cs in the presence of ABA (Fujii et al., 2009; Ma et al., 2009b; Park et al., 2009). Structural studies confirmed that the PYLs are ABA receptors and found the PP2Cs can function as co-receptors because they enhance the ABA-binding affinities of the PYLs (Santiago et al., 2009; Yin et al., 2009). In Arabidopsis protoplasts, PYR1 and PYL1-12 relieved PP2C (ABI1) inhibition of ABA-dependent activation of RD29B-LUC expression by the SnRK2 protein kinases such as OST1/SnRK2.6 (Fujii et al., 2009).
PYLs have an ABA-binding pocket comprised of four highly conserved regions named CL1-4 (Yin et al., 2009). The structure of PYL changes after ABA binding (Santiago et al., 2009; Yin et al., 2009), allowing the formation of an ABA-PYL-PP2C complex and the inhibition of PP2Cs, thus releasing the inhibition of SnRK2.2/2.3/2.6 by clade A PP2Cs. SnRK2.2/2.3/2.6 are positive regulators of ABA signaling. Activated SnRK2.2/2.3/2.6 can phosphorylate and activate transcription factors such as ABFs/AREBs to induce the expression of ABA-responsive genes such as RD29B (Fujii et al., 2009). In guard cells, the SnRK2s can also phosphorylate and activate the NADPH oxidase catalytic subunit RBOHF, leading to the production of reactive oxygen species (ROS).
The interactions between PYLs and PP2Cs are highly specific. Many monomeric PYL proteins may also interact with certain PP2Cs in an ABA-independent manner in yeast two hybrid assays and pull down assays (Hao et al., 2011; Park et al., 2009). However, this interaction is weak and in the particular case of PYL10-ABI1 interaction, the dissociation constant (Kd) changed from 1.2 μM without ABA to 0.02 μM with ABA (Hao et al, 2011), indicating that the interaction between PYL10 and ABI1 is enhanced by ABA. In vitro assays found that PYL10 shows obvious ABA-independent inhibition of ABI1, HAB1 and HAB2 with a PYL:PP2C ratio of 1:1, and PYL5-10 (except the untested PYL7) show ABA-independent inhibition of ABI1, HAB1, HAB2, and PP2CA to different degrees with the PYL:PP2C ratio of 10:1 and 100:1 (Hao et al., 2011). However, the ABA-independent inhibitions are much weaker than ABA-dependent ones.
Although the PYL family proteins have been studied intensively in recent years, little information is available for PYL13, which differs from other PYLs in the highly conserved regions of their ABA-binding pocket.